The present invention relates to a power transmission device for a vehicle, and, more particularly, relates to a power transmission device for a vehicle with two axially opposed driven wheels, in which a differential is provided between these driven wheels and is axially located closer to one of these wheels and further from the other of them--i.e. in an asymmetrical location relative to the wheels.
In the case of a vehicle provided with a transversely mounted engine, especially in the case of a vehicle of the front engine front wheel drive type or the so called FF type, it is very common for the differential device which transmits power from the gearbox of the vehicle to the driven wheels thereof, and which provides differential action between said driven wheels, to be displaced to one side or to the other side of the longitudinal axis of the vehicle, i.e. for this differential device to be closer to one of the driven wheels and further from the other of the driven wheels. This is because of the limited space available in the engine compartment of such a vehicle, due to the transverse mounting of the engine therein, and to the relative alignment of the various units in the power train of the vehicle, and is a well known matter in the automotive art.
The question then arises as to how the rotary power is to be transmitted from such an asymmetrically positioned differential to these driven wheels of the vehicle. If the differential is provided with left and right power output shafts which are of the same length, and if the end of each of these power output shafts is connected to its respective driven wheel by a drive shaft and by universal joints, then, since the left and the right drive shafts will necessarily be of different lengths, these drive shafts will bend downwards at different angles, and will be connected to the driven wheels through different angles. This means that during acceleration of the driven wheels, and also, when the driven wheels are the front wheels of the vehicle and are also used for steering the vehicle, during turning round corners, then the difference in the torques provided to these driven wheels of the vehicle, when the steering geometry changes as it inevitably will in such circumstances, will cause asymmetrical acceleration and steering characteristics for the vehicle. For example, it may be the case that a tendency should arise for the vehicle, during acceleration, to steer towards the side on which the drive shaft is shorter. This deteriorates straight ahead drivability of the vehicle, and accordingly is quite unacceptable.
In view of the above described problem, a constructional solution has been proposed for a power transmission with such an asymmetrically located differential, in which the left and right drive shafts are of equal lengths, and bend downwards at the same angles, and on the side of the vehicle on which the distance between the differential and the driven wheel is the shorter the inner end of the drive shaft is directly connected to the outer end of the power output shaft of the differential by a universal joint, while on the side of the vehicle on which the distance between the differential and the driven wheel is the greater the inner end of the drive shaft is connected to the outer end of an intermediate shaft, the inner end of which is connected to the outer end of the power output shaft of the differential on that side. In this case, this intermediate shaft is supported, via bearings, by some fixed member of the vehicle.
In such a construction, the angles down through which the left and the right drive shafts incline are the same, and accordingly the left and right driven wheels are provided with equal steering torques, accordingly eliminating asymmetrical steering and driving characteristics of the vehicle; on the other hand, because the power output shaft of the differential on the side on which the differential is further from its driven wheel needs to be connected to the intermediate shaft via a third universal joint, in order to compensate for manufacturing tolerances in the alignment of the rotational axis provided for the intermediate shaft by the means for supporting the intermediate shaft, and the rotational axis of the power output shaft on that side of the differential, thereby the number of mechanical components is increased, and accordingly the cost of the power transmission is increased and the difficulty of manufacture thereof becomes higher. Further, the reliability of the transmission is decreased, and further problems arise such as an increase in weight, and an increase in the slack in the power train caused by the additional splined joints in the power train. Accordingly, such a solution, although it has been practiced, is by no means perfect.
Therefore, in order to cope with these problems, it has been proposed for no such intermediate shaft to be used, but instead for the power output shaft of the differential on the side where the differential is further from its driven wheel to be longer than the power output shaft of the differential on the side where the differential is closer to its driven wheel, so that on both sides the power output shafts of the differential are directly connected to their appropriate drive shafts by universal joints, both of these drive shafts being of the same lengths and angling downwards at the same angles. This solution is effective for minimizing the number of universal joints in the power train, and for ensuring that no asymmetrical steering characteristics or driving characteristics of the vehicle are caused. However, in order to provide sufficient rigidity for the support of the extended power output shaft of the differential on the side where the differential is further from its driven wheel, it is necessary for this long power output shaft to be rotatably supported near its outer end. In the past, this has been done by supporting the outer end of this long power output shaft by a bearing device which has been supported by a bracket mounted to the engine block of the vehicle. A problem has therefore arisen in this prior art, in that, since the central axis of the long power output shaft must be maintained as passing through the power output axis of the differential, in order to avoid rapid wear on the internal parts of the differential, a high dimensional accuracy has been required for the aforesaid bracket mounted to the engine block, and in practice during assembly of the power train of the vehicle an adjustment operation has been necessary. This has been troublesome and costly, and further has caused servicing problems for the vehicle during use, which have sometimes deteriorated the reliability of such a transmission. That is, if the transmission and/or the engine of such a vehicle require to be removed from the vehicle for major servicing, or for replacement or the like, then, when the vehicle is being reassembled, it is necessary to readjust the aforesaid bracket, in order to provide for proper alignment of the long power output shaft. This is difficult and troublesome, especially for a workplace or garage which is not very fully equipped; and of course, if this adjustment operation is not carried out, or is carried out improperly, then quick deterioration and eventual failure of the transmission will inevitably result, due to misalignment of the parts thereof.